1. Field of the Invention
The present invention relates to a stator and a ceramic tube type ultrasonic motor using the same. In the stator, an elastic body producing friction with a rotor is formed of a ceramic material having excellent abrasion resistance instead of a metallic material, thereby minimizing abrasion caused by the friction.
2. Description of the Related Art
As the information technology rapidly develops, electronic and optical apparatuses are required, which are faster and more precise. Particularly, the market of portable mobile application products is expanding every year. Among them, mobile phones having a camera mounted thereon tend to adopt a function of digital camera such as autofocusing or optical zoom, as the number of pixels of a sensor increases. Makers focus on developing in order to preoccupy the market.
As an apparatus for driving a camera module for mobile phone, a stepping motor, a VCM, and an piezoelectric ultrasonic motor are representative. In order to implement a rotation motor function, the piezoelectric ultrasonic motor converts simple vibration, such as contraction and expansion, into circular or linear movement by using the friction between a stator and a rotor, the vibration being generated when an electric current is applied to piezoelectric ceramic. The piezoelectric ultrasonic motor has higher energy density, faster response speed, and higher position precision than an electromagnetic driving motor. Further, the piezoelectric ultrasonic motor having an off-power holding function does not make noise at the time of operating and is not affected by an electromagnetic wave.
General characteristics of driving apparatuses for driving a lens of camera module for mobile phones are summarized as in the following Table 1.
TABLE 1Voice coil motorDC motorStepper motorPiezo motorSizesmallmediumlargesmallPricelowmediumhighlowSpeed20 ms40 ms100 ms3 msWhether to drivepossible (springpossiblepossiblepossiblein both directionsstructure)RepetitionhighlowmediumlowDeceleratordoes not havehavehavedoes not haveApplicationAF and on/offAF and zoomAF and zoomAF and zoomfunction
Meanwhile, the performance of the friction-drive-type piezoelectric ultrasonic motor is affected by a material of the stator and rotor composed of an elastic body and piezoelectric ceramics. Particularly, when an elastic body of the stator is selected, the ease of processing and the abrasion resistance should be considered.
As the most representative motor, the piezoelectric ultrasonic motor invented by T. Sashida in 1980 has used serrated metal (brass-based) as a material of an elastic body, in order to expand the displacement of piezoelectric ceramics. In such a piezoelectric ultrasonic motor, however, characteristics thereof are degraded by the friction with a rotor, and a driving frequency changes. Particularly, since foreign substances having serious influence on a camera module are generated, a material having high hardness is attached between the stator and rotor or coating is carried out, in order to minimize the foreign substances.
As the related arts regarding the tube type piezoelectric ultrasonic motor, there are provided ‘Mechanism comprised of ultrasonic lead screw motor’ disclosed in US Patent No. 2005-0052098 and ‘An ultrasonic motor using vibration of a short cylinder’ disclosed in ‘IEEE Transaction on Ultrasonic, Ferrorelectric, and Frequency Control, Vol. 36, No. 5, pp. 517-521’ in 1989.
In the piezoelectric actuator disclosed in US Patent No. 2005-0052098, piezoelectric ceramic is attached on a metallic housing corresponding to an elastic body of the present invention, or a piezoelectric tube with four outer electrodes is contained. As described above, however, the housing can be worn down because the metallic elastic body is formed in a tube shape. Further, when the tube is made of piezoelectric ceramic, there are difficulties in manufacturing, and a driving force is not effectively delivered to a rotor because of a low friction index and low mechanical strength of piezoelectric ceramic.
In the ultrasonic motor disclosed in the above paper, a Langevin vibrator used as a driving source is constructed so that metallic bodies are fixed to both sides of two piezoelectric ceramics by bolts. The metallic bodies are formed of stainless steel in order to amplify vibration of the piezoelectric ceramics, and a friction material is inserted therein in order to prevent the abrasion caused by the friction with a rotor formed of brass. In this case, however, the above-described abrasion can still occur, because the metallic bodies serving as an elastic body of a stator are made of a metallic material such as stainless steel.